1. Field of the Invention
The present invention relates to an adaptive delta modulation system which automatically corrects the crossing between a coder terminal and a decoder terminal of a delta modulation signal occurring in the adaptation logic circuit of the system.
2. Description of the Prior Art
Within a conventional adaptive delta modulation system, in the coder terminal, an analog input signal is supplied to the first input terminal of a comparator of an adaptive coder, the output of the comparator is fed back via an integrator circuit to a second input terminal of the comparator so as to compare the output of the integrator circuit with the analog input signal, and determine whether the output digital coded signal "0" or "1" from the comparator, in accordance with whether output of the integrator circuit is either larger or smaller than the analog input signal. The output of the comparator is supplied to the adaptation logic circuit. The adaptation logic circuit examines whether the continuous bit outputs of the comparator remain constant (that is: 0 or 1) or not, and discretely changes the quantum stepsize in accordance with the companding law and initiates the signal output of the adaptive delta modulation. In the decoder terminal, the received signal of the adaptive delta modulation is supplied to a adaptation logic circuit, which is similar to that in the coder terminal, except for the carrying out of an inverse companding function, and the output of the adaptation logic circuit is supplied via an integrator to a low pass filter which produces a demodulated analog output signal.
In the above-mentioned conventional system, it is well known that sufficient dynamic range can be obtained when the companding law and a suitable number of different stepsizes is selected. However, one problem is that, when the signal is demodulated; the initial conditions of the coder and decoder terminal are not coincident. Therefore, if the initial conditions of the coder and decoder terminals are not preset, the adaptation logic circuit tracking is delayed between the coder terminal and decoder terminal, that is, a mis-tracking occurs and normal functioning of the system can not be expected. If the adaptative logic circuit in the coder and decoder terminals are operated normally, mistracking is due to a faulty transmission line. The methods of overcoming the above-mentioned drawback are (i) to transfer to a minimum stepsize after idle channel state or (ii) to transfer to a maximum stepsize in the case of overload. However, in the normal functional state, the probability of the maximum stepsize being realized is very small, and the function in the idle channel state is not realized when background noise exists or a data signal is transmitted via an acoustic coupler. Therefore, abovementioned mistracking should be corrected by using some other suitable means.
For the purpose of eliminating said mistracking a method has been proposed which provides, beside the transmission line for the adaptive delta modulation signal, a low speed transmission line for sending information concerning the state of the adaptation logic circuit in the coder terminal and for correcting mistracking, in the decoder terminal. This method can eliminate mistracking completely, however another transmission line is required and this complicates the apparatus somewhat.
An alternative method is proposed in which the stepsize of two bits of the same kind (that is "0" "0" or "1" "1" ), is raised one stepsize, or if said two bits are of a different king (that is: "0" "1" or "1" "0" ), the stepsize is dropped to the minimum stepsize. In this method, the adaptation logic circuits in the coder terminal and the decoder terminal transfer to the minimum stepsize every time successive different signals are produced and mistracking can be corrected automatically. However, in this method, sufficient dynamic range can not be obtained, for example, when the voice signal is transmitted.